The red circles aren't merchies - those are actually aging sonar tracks - indicating roughly where the target could be since you lost continuous sonar contact with it. Any time you pick something up on sonar then lose it, these circles start 'growing'. So the circles there there are an old return from the sub (not yet merged with the main track), and probably a bunch of shrimp and trawlers further south picked up by the same pings but never followed on.
So far so good - managed to hit over 20,000pt on the night mission with the Shark, but it was all due to that quick dash, followed by me sticking to the SSK like a leech while dodging the merchants and fishing boats only slightly...
